814 Transactions of the Society. 



delicate organic tissues or other similar objects, but it will show far 

 too much colour, and, therefore, a great lack of definition when it 

 is used on diatoms or other preparations requiring oblique illumi- 

 nation ; and on the other hand a lens which has been made for the 

 brilliant exhibition of diatoms may possibly be nearly useless for 

 histological work. 



This divergence would not be of much importance if objectives 

 of especial excellence could be constructed for the one or for the 

 other kind of work. But this is not the case, because the operation of 

 the different zones of aperture is not really separated by difierent 

 modes of illumination. Though the incident pencil may be either 

 central or in the marginal zone, deflected and diffracted pencils will 

 generally engage very different parts of the aperture, except in 

 certain special preparations. Consequently the only rational plan 

 for general work must be so to distribute these deviations as to 

 prevent their accumulation in one joart of the aperture. This is 

 done by correcting the spherical aberration of the rays of predomi- 

 nant intensity (the yellow rays for visual performance), and com- 

 pensating the greater part of the residual aberration of the red and 

 blue by a moderate amount of chromatic under-correction — the plan 

 now adopted, consciously or unconsciously, by the best opticians. 

 But by the best possible distribution of these deviations they cannot 

 be eliminated, but must always remain in the image, and to a certain 

 extent debase the general level of the defining power. 



In the year 1873 I expressed an opinion that the correction 

 defect in question is the principal objection to the use of objectives 

 of moderate focal length with deep eye-pieces, instead of the deeper 

 lenses now required.* 



It is now sixty years since C. F. Gauss, the celebrated mathe- 

 matician of Gottingen, considered the above-mentioned aberration 

 defect. He then discussed the problem of a more perfect correction 

 of objectives, and suggested a plan for getting rid of the residual 

 aberration in binary lenses made of ordinary crown and flint, for 

 telescojjic use. Unfortunately, his plan, which is disadvantageous 

 even for telescopes, is utterly inapplicable to systems having the 

 large apertures required in Microscopes. His method of correction, 

 (without considering other difficulties) would need uncemented 

 concave lenses of curvature so deep that even moderate apertures 

 would make the angles of incidence approach, or exceed, the limit 

 of total reflection from glass to air. 



My own researches upon the subject, beginning from 1870, 

 and having special regard to the Microscope, had for a long time a 

 negative result — as far as the actual conditions of optical work 

 were considered. From various practical difficulties every i^lan 



* ' Beitriige z. Theorie d. Mikruskops.' ' Arohiv mikr. Anat.,' Bd. ix. p. 425. 



